The use of optical brighteners, also known as fluorescent whitening agents, in laundry detergents is desirable from an overall performance standpoint. Brighteners deposit onto fabric surfaces where they absorb ultraviolet radiant energy, such as that found in ordinary daylight, and reemit the energy as a blue light which reduces or eliminates any yellow cast to fabrics and gives them a brighter appearance.
The selection of suitable brighteners for detergents containing nonionic and cationic surfactants presents a special problem since many conventional brighteners are anionic in nature and tend to form insoluble complexes with the cationic surfactants, thereby decreasing the effectiveness of both brightener and surfactant. This problem has been recognized in the art and a number of potential solutions to it have been suggested. For example, U.S. Pat. No. 2,742,434, Kopp, issued Apr. 17, 1956, U.S. Pat. No. 3,904,533, Neiditch et al, issued Sept. 9, 1975, and Japanese laid-open publication No. 43708/78, Kao Soap Company, published Apr. 20, 1978, teach the use of specifically selected anionic brighteners for use in cationic surfactant-containing detergent compositions. Further, certain types of nonionic and cationic brighteners have been suggested in U.S. Pat. No. 3,704,228, Eckert et al, issued Nov. 28, 1972, U.S. Pat. No. 3,896,034, Eckert et al, issued July 22, 1975, and South African Application No. 65/5106, General Foods Corporation, published March, 1966. While many of these brighteners are compatible with certain types of cationic surfactants, their usage over time tends to discolor fabrics, generally with a greenish/yellow tinge, making them impractical for use in commercial laundry detergent compositions. Cationic brighteners in particular tend to deposit onto soils and cause greenish/yellow tinting of fabrics. In addition, some nonionic brighteners are not suitable because they tend to bioaccumulate in the environment.
It has now been found that by selecting the specific anionic brighteners herein for use in cationic/nonionic detergent compositions, excellent brightening performance is achieved, particularly on cotton fabrics, without any concomitant discoloration problems. While not intending to be limited by theory, it is believed that the anionic brighteners herein are highly effective because they are extremely soluble in the present nonionic/cationic surfactant systems and do not readily form insoluble complexes with the cationic surfactants.